The Indian Ocean is the third largest ocean and includes four distinct sub regions: Western Indian Ocean, Central Indian Ocean, Middle Eastern Seas and Southeast Asia It is closed to the north, and a large proportion of its waters are tropical or near tropical. Unlike the Atlantic it is largely bounded by relatively arid countries and does not receive particularly high inputs of freshwater or terrestrial sediments. The great exception to this is the Bay of Bengal in the northeast , which is fed by massive riverine discharge by a number of rivers, leading to conditions of high sediments and low, fluctuating salinities -conditions inimical to coral reef development. To the northwest are the enclosed sea areas of the Red Sea and the Arabian Gulf, with very different tectonic histories, but both occurring in  highly arid regions with little terrestrial runoff. The coast of East Africa is also relatively dry. Continental shelf areas are generally narrow, although there are a few near shore island groups which are important for coral reef development. There are also several oceanic island groups, notably in the west and central parts of this ocean.

There are large areas of coral reef right across this region, making up nearly twenty percent of the world total. Fringing reefs predominate along much of the Red Sea, particularly northern and central parts. Further south in the Red Sea and Arabian Gulf coastal sediments and high salinities restrict fringing reef development, though there are extensive offshore patch reefs. Cool upwellings limit the development of true reefs along parts of southern Arabia and Pakistan. Further south there are fringing communities on the coasts of East Africa, and particularly along the shores of the continental islands. Some of the best developed reef structures occur in isolated oceanic locations. There are numerous atolls and platform structures in the west and central regions of the oceans, and the Maldives and Chagos Archipelago include the largest atoll structures in the world. The continental coastlines of India and Sri Lanka have very limited reef development as there are various adverse conditions, including high turbidity, fluctuating salinity and high wave energy. There are important though little known reefs around the Andaman and Nicobar islands and to the north of Sumatra. Australia also has significant reef communities, including extensive fringing reefs, offshore platforms and barrier structures and high latitude communities.

Species diversity is high across the region, following a narrow band of high diversity in the Central Indian Ocean and forming two distinctive sub-centers of diversity in the Western Indian Ocean and the Red Sea. Elsewhere there is greatly reduced diversity, notably in the Arabian Gulf and along the shores of mainland India. Despite their high latitude there is relatively little diminution of diversity in the reefs of the northern Red Sea. By contrast there are latitudinal declines in species numbers, and as a consequence in the development of reefs themselves, in both southern Africa and Western Australia.

Wide areas of this region were affected by the 1997-98 El Nino Southern Oscillation event, and in 1998 warm waters swept across wide areas of the Indian Ocean, leading to bleaching and massive levels of coral mortality on reefs from Western Australia to the shores of East Africa.  Direct human pressures on coral reefs in the Indian Ocean are highly varied. The Arabian Gulf contains the largest concentrations of oil reserves in the world, and there is chronic oil pollution in this sea, exacerbated by occasional massive oil spills. Tanker traffic also carries the threat of oil pollution to other areas, notably the narrow straits at the mouth of the Arabian Gulf, and at the northern and southern ends of the Red Sea. Coastal development is sporadic -there are vast areas of the Arabian coastline with little development, but in others, such as around the major ports and some of the tourist areas of East Africa, coastal development is having a direct impact on reefs. Tourism is sporadic, but is critical to the economies of Egypt, Kenya, Tanzania and the islands of the Indian Ocean.

Southeast Asia is one of the most important areas in the world for coral reefs. Over thirty percent of the world's reefs are found in this complex region which straddles the waters between the Indian and Pacific Oceans. The region includes the continental coastlines of Myanmar and Thailand, Malaysia, Cambodia, Vietnam and China. The most extensive coastlines, however, follow the great complex of islands which are dominated by the Philippines and Indonesia. Indonesia alone has over 50,000 square kilometers of coral reefs, nearly eighteen percent of the world total. Japan lies on the edge of this region, and also has a considerable area of reefs surrounding island chains which follow natural clines of diminishing species diversity. The coastlines define a large number of partially enclosed seas. While the waters around the continental shores are generally shallow, deep oceanic waters come in close contact with offshore islands in many areas.

Fringing reefs predominate, although there are also extensive barrier reef systems, and a number of atolls and near-atolls. This region is the great center of coral reef biodiversity, and there are more species here, in almost all animal groups, than anywhere else. To some degree this diversity is encouraged and maintained by the complexity of the coastline and the great range of habitats found in the region, but its ultimate origins can be traced back over geological timescales. While extinctions were occurring in other regions it would appear that species were able to survive in this region, and even to diversify as sea levels fluctuated and areas became isolated and then reconnected with one another.

Unfortunately this region is also the most threatened and disturbed by human activities. Some eighty-two percent of the region's reefs were considered to be threatened by human activities in the 1998 Reefs at Risk Study. Most of these threats are linked to the rapidly growing economies and populations in this region. These are driving massive changes in the landscape, with forest clearance and agricultural intensification leading to increased sedimentation and pollution from agricultural chemicals. Massive urban expansion has also led to enormous pollution problems close to urban centers. Fishing pressures are ubiquitous, from chronic over fishing for local consumption and the highest rate of blast fishing (fishing with explosives) in the world, to target species over fishing even in many of the remotest parts of the region.

Extracted from The World Atlas of Coral Reefs, by Mark D. Spalding, Corinna Ravilious and Edmund P. Green, published by the University of California Press . For more complete and in-depth coverage of the topics presented in this webpage, I recommend highly purchasing a copy of this beautifully illustrated book. Just click on the University of California Press link above to do so.